How does exposure work for KA mount lenses

Hi everyone,

I'm hoping someone can explain to me how a DSLR camera calculates exposure when using a "Pentax-A" KA mount lens.
I have a Pentax-KR and am using a "Pentax-A" 50mm f2 lens.

The below is my understanding of various configurations.

Using modern KAF2 lens with contacts on the lens
The lens contacts communicate the minimum and maximum aperture settings of the lens to the camera. At half-press of the shutter release the camera reads the light level at the "viewing" aperture (i.e. wide open) and can therefore calculate the required exposure for the "taking" aperture (i.e. stopped down to required f-stop) when the actual shutter is released.

Using "Pentax-A" KA mount lens set at a specific aperture
When an aperture is set on the lens, the camera doesn't know what aperture the lens is set at. Therefore with the camera set to Manual (and the "Green Button" function "Action in M Mode" set to "Tv Shift" ) to take the exposure you first need to press the Green Button. This stops the aperture down to the "taking" aperture, reads the light level, and adjusts the shutter speed for you.

Using "Pentax-A" KA mount lens set to Auto "A" aperture
When the aperture is set to "A" on the lens, the camera controls the lens aperture. Light reading is taken at "viewing" aperture on half-press of the shutter release button. The exposure for the "taking" aperture is calculated based on the difference between the "viewing" and "taking" aperture.

Question is: For "A" aperture, how can the camera calculate the exposure if the "viewing" aperture value is not known?

I was reading up about the contacts in the following http://kmp.bdimitrov.de/technology/K-mount/Ka.html and thought maybe that since the camera's contacts would all be touching the bare metal of the KA mount it would register as a lens contact pattern 111x11 which relates to an f1.2~f22 lens. Does this mean my camera thinks my lens is f1.2 wide-open (1.5 stops faster than my actual lens)?

I took few test shots and compared setting a specific aperture on the lens with setting the "A" lens aperture and setting the camera to the same aperture. There does seem to be a difference, typically the "A" aperture gives about 1 stop less exposure.

What do you think? Do others have differences between manual and auto settings? Am I barking up the wrong tree?

Cheers,

With an A lens the camera does know the viewing aperture, just as it does with the KAF2 lenses. They use the same contacts.

It's just that if the lens is set to anything other than the 'A' setting, the camera has no way of knowing what aperture it's set at.